1. Field of the Invention
The present invention relates to a heating furnace for heating bodies by fuel combustion flames, and particularly to a heating furnace suitable for use in a continuous steel slab heating furnace and having a high heat transfer efficiency for the bodies to be heated.
2. Description of the Prior Art
In a heating furnace in which bodies are heated by fuel combustion flames, particularly, in a continuous steel slab heating furnace, heat transfer to steel slabs is effected directly through radiation heat transfer and convection heat transfer from combustion gas, and indirectly through radiation heat transfer from the refractory material on the furnace wall heated through radiation and convection heat transfer from combustion gas.
An example of the conventional four-zone steel slab heating furnace as described above is shown in FIG. 1. The upper portion of the drawing shows the construction of the heating furnace, and the lower portion shows the distribution of temperature in the heating furnace. Referring to the drawing, designated at 10 are steel slabs being continuously charged through an inlet 12 for charging and sent out through an outlet 14 for extracting; 16 an upper heating zone where axial flow type burners 18 are disposed for blowing out fuel combustion flames 17 in parallel to the moving direction of the steel slabs; 20 a lower heating zone where also axial flow type burners 18 are disposed; 24 on upper soaking zone where also axial flow type burners 18 are disposed; 28 a lower soaking zone where side burners 30 are disposed for blowing out fuel combustion flames perpendicularly to the moving direction of the steel slabs; and 32 a waste gas exhaust port. Additionally, the inner wall surface of this heating furnace is entirely covered by refractory material.
Now, if steel slabs 10 each having a given value of thickness are heated under conditions that the heating load is M ton/hour; the heat pattern (wall temperature pattern) in the furnace H.sub.1, and the temperature pattern of combustion gas G.sub.1, then the curves of temperature rise of the steel slabs are shown by solid lines .theta..sub.S1 (surface temperature) and .theta..sub.C1 (center temperature) in FIG. 1. The curves of temperature rise of the steel slabs under the same conditions as above except that the heat pattern in the furnace is H.sub.2 and the temperature pattern of combustion gas G.sub.2 are shown by broken lines .theta..sub.S2 (surface temperature) and .theta..sub.C2 (center temperature) also in FIG. 1. Consequently, in a section from Point O, the inlet for charging to Point x, the inlet of heating zone, H.sub.1 &gt;H.sub.2, .theta..sub.S1 &gt;.theta..sub.S2 and .theta..sub.C1 &gt;.theta..sub.C2, in a section from Point x described above to Point y, the outlet of heating zone, H.sub.1 &lt;H.sub.2, .theta..sub.S1 &lt;.theta..sub.S2 and .theta..sub.C1 &gt;.theta..sub.C2, and at point y, .theta..sub.S1 =.theta..sub.S2, .theta..sub.C1 =.theta..sub.C2, whereby heating is effected at the same temperature in both cases. In addition, if the same fuel is used and the same excess air ratio is adopted, the temperature of combustion gas is the same, i.e. G.sub.0, at the burner portion, and thereafter G.sub.1 &gt;G.sub.2 and .theta..sub.g1 &gt;.theta..sub.g2 at Point O of the furnace end. In the heating furnace as described above, the loss of heat caused by waste gas exhausted from a waste gas exhaust port 32 disposed adjacent to the inlet for charging of the heating furnace is large. Hence, in comparison between the heat patterns H.sub.1 with H.sub.2, the latter has less heat loss and better thermal efficiency than the former. Consequently, with the conventional heating furnaces, the heat pattern in the furnaces have been made to be close to the heat pattern H.sub.2 in designing the burners or the configurations of furnaces. However, in the case the kinds of fuel, the burners, the configurations of furnaces and the excess air ratio are the same there has not existed any means for further raising the temperature at the central portion of the heating zone.
On the other hand, heretofore, convection heat transfer from the high temperature and high speed flow of fuel combustion flames has not been positively utilized.